In many species, sperm binding to oviduct epithelium is believed to be an essential step in generating a highly fertile capacitated sperm population primed for fertilization. In several mammalian species, this interaction is based on carbohydrate-lectin recognition. d-galactose has previously been characterized as a key molecule that facilitates sperm–oviduct binding in the horse. We used oviduct explant and oviduct apical plasma membrane (APM) assays to investigate the effects of various carbohydrates; glycosaminoglycans; lectins; S-S reductants; and the capacitating factors albumin, Ca²⁺ and HCO₃ ⁻ on sperm–oviduct binding in the horse. Carbohydrate-specific lectin staining indicated that N-acetylgalactosamine, N-acetylneuraminic acid (sialic acid) and d-mannose or d-glucose were the most abundant carbohydrates on equine oviduct epithelia, whereas d-galactose moieties were not detected. However, in a competitive binding assay, sperm–oviduct binding density was not influenced by any tested carbohydrates, glycosaminoglycans, lectins or d-penicillamine, nor did the glycosaminoglycans induce sperm tail-associated protein tyrosine phosphorylation. Furthermore, N-glycosidase F (PNGase) pretreatment of oviduct explants and APM did not alter sperm–oviduct binding density. By contrast, a combination of the sperm-capacitating factors albumin and HCO₃ ⁻ severely reduced (>10-fold) equine sperm–oviduct binding density by inducing rapid head-to-head agglutination, both of which events were independent of Ca²⁺ and an elevated pH (7.9). Conversely, neither albumin and HCO₃ ⁻ nor any other capacitating factor could induce release of oviduct-bound sperm. In conclusion, a combination of albumin and HCO₃ ⁻ markedly induced sperm head-to-head agglutination which physically prevented stallion sperm to bind to oviduct epithelium.

Previous studies have shown that a soluble protein fraction derived from preparations of apical plasma membrane (APM) of the oviductal epithelium enhances the in vitro survival of mammalian spermatozoa. Here, we show that the survival enhancing property of the soluble protein fraction seems to depend significantly upon heat shock 70 kDa protein 8 (HSPA8 previously known as HSPA10). The following findings in the present study enabled us to draw this conclusion: first, using proteomic analysis, we identified a subset of 70 kDa oviductal surface proteins that bound to spermatozoa, one of which was HSPA8. Second, pre-treatment of the soluble protein fraction with anti-HSPA8 antibody reduced the 24 h (at 39 °C) sperm survival enhancement effect normally induced by the presence of 200 μg/ml soluble APM proteins. Third, complementary experiments showed that substituting the soluble protein fraction with bovine recombinant HSPA8 (0.5–2 μg/ml) also elicited the sperm survival effect. Finally, we also tested the effect of bovine recombinant HSPA8 on bull spermatozoa and found similar, dose-responsive, sperm survival promoting effects. The conserved nature of HSPA8 between mammalian species suggests that this protein may represent a common biological mechanism for the maintenance of sperm survival in the oviduct.